Many types of fluid flow control devices are known. One such device in widespread service is known as a spool valve. A spool valve generally comprises a housing defining a cylindrical bore having a valve member slidably disposed therewithin. The valve is opened or closed by changing the position of the valve member in relation to inlet and outlet ports formed in the housing. Some spool valves are used to produce variable flow volumes in response to a varying input signal. Such devices may be known as proportional valves or throttle valves. Other spool valves are used only to fully open or fully close a flow path in response to an input signal. Such devices may be known as fluid switches. One application for fluid switches is to serve as a fuel cut off valve in the fuel system of a vehicle.
In many applications, especially those in which the fluid switch will be incorporated into a low cost or expendable unit, it is desirable to make the switch both reliable and inexpensive. Experience has shown that the reliability of a device is often related to the number of components it contains, especially the number of moving components. Experience has also shown that the cost of a device is often related to the number of components it incorporates, to the complexity of fabricating these components, and to the complexity of assembling the components into a finished device. A need therefore exists, for a fluid switch having a small number of components, especially moving components, and where these components are simple to fabricate.
In many applications, it is desirable that the fluid switch maintain a given flow state (i.e., "on" state with full flow or "off" state with flow cut off), in the absence of a continuous control input. Various types of locking mechanisms or feedback circuits are known in the prior art to maintain a given flow state, however, these features typically add to the number and complexity of the parts in each unit, thereby adversely influencing the cost or reliability of the device. A need therefore exists, for a fluid switch that maintains a given flow state in the absence of a continuous control input, yet has only a small number of components which are simple to fabricate.
In many applications, a fluid switch is provided with a means for positioning the valve member so as to select the desired flow state. Solenoids are often used for these positioning means as they provide a ready method for transforming an electrical control signal into the mechanical input needed to position the valve means. The application of solenoids for the positioning of spool valves is well known, however, in many applications a constant current is required through the solenoid core to maintain a fluid switch in a given flow state. Maintaining a constant current through a solenoid coil requires an increased use of electrical power and may complicate the design of control circuitry compared to a "single pulse" control input. A need therefore exists for a solenoid operated fluid switch that requires only a single pulse to change flow states and does not require constant current to maintain a given flow state.